Greening the blacktop
Motorways and trunk roads make up only about 3% of the road network in Great Britain
Asphalt is a sticky subject in more ways than one. Great Britain has around 395,000km of highway, and more than 95% of it is paved with asphalt. Less than 3% of the road network - the motorways and trunk roads - is managed by National Highways; the rest is the responsibility of local authorities.
And almost all rely on carbon-heavy materials for their pavements. When National Highways calculated its carbon emissions baseline in 2020, asphalt was found to be responsible for a significant contribution, around 15%, hence it is a key target for the agency in its bid to achieve net zero emissions from construction and maintenance activity by 2040. Luckily, asphalt also has great potential for carbon savings, whether that is by finding substitutes for the fossil-fuel-based binders that hold the material together, using low-carbon fuels to make and place it, enabling greater quantities of asphalt to be recycled into new paving, or improving durability of the surfacing so it does not need to be replaced as often.
The quest for low carbon asphalt surfacing is driving a National Highways funded research programme that is tasked with examining new technologies. The aim is to establish their potential contribution to reducing asphalt's carbon footprint and to find out how quickly they can be brought into use. This article was first published in the November 2024 issue of The Construction Index Magazine.
Sign up online.[1] Local authorities are also chasing carbon reduction targets through the Live Labs 2 programme, which has led to the establishment of the Centre of Excellence for Decarbonising Roads.
Both have similar high-level aims and are working to the same timetable, but the priority for local authorities tends to be reliable pot-hole repairs, while National Highways is seeking low-carbon surfacing and improved durability for high-speed roads. National Highways' head of specialism for pavements in the safety engineering & standards team, Paul Edwards, says they wanted a research process that would enable any promising innovations to become 'business as usual' quickly enough to make a difference.
An extensive review at the start of the programme sifted the products and innovations that might make a useful contribution to lowering carbon in asphalt, and assessed what effort would be needed to bring them to the point of use. Researchers were looking for technologies that had already been tested or proven 'off-network' (for example on local authority roads, or in another country) and were ready to be trialled 'on-network'. But rather than testing and developing each innovation separately, the approach has been to 'stack' or combine technologies to try and maximise savings in one hit - perhaps by testing a new bio binder in a mix that also includes a higher percentage of recycled asphalt.
The low-carbon asphalt research is being led by consultant AtkinsRealis and Nottingham University. AtkinsRealis technical director Donna James explains: "The first part of the project was to try and understand what type of innovations are out there, what level of technology readiness they had and also, importantly, looking at how they might fit into the current standards and specifications. "We are trying to take a holistic view, rather than the usual 'linear' approach of doing some lab testing, then some road trials, and then finally start to think about standards and specifications.
We wanted to address that right from the start, to accelerate the uptake of these technologies," explains James. One of the challenges is to narrow down the range of options. "There are lots of variables - additives, additives with recycled asphalt, additives with recycled asphalt and lower temperature mixes. The early work identified that there wasn't going to be one single solution and so the best way to achieve real leaps towards net zero was to stack these technologies together and use all the levers at once," says James.
And lowering carbon emissions in the production and laying of asphalt is not a standalone goal. "We do need to ensure that the material remains safe and durable. There's no point in achieving lower carbon in the first stage if that leads to reduced durability - then your whole-life carbon would increase," James explains.
Conversely, other additives might not reduce the carbon of the material as it is produced and laid, but if they improve durability and extend the lifetime of the surfacing, then whole-life emissions are reduced.
There is also a push to use a higher percentage of recycled asphalt in new pavements, usually by incorporating rejuvenator additives. The average life of surfacing on network roads is currently 12 years, says Edwards, "although that doesn't mean it's maintenance-free in that period." He adds: "Our 2040 asphalt roadmap helped us identify priorities for the research.
There are two hotspots for carbon in asphalt - one is the use of bitumen as a binder, the other is the energy used to make it." This article was first published in the November 2024 issue of The Construction Index Magazine. Sign up online.[2]
The five main aims are to maximise the durability of pavements, adopt lower carbon fuels in the production of asphalt, make it at lower temperatures, use a higher percentage of reclaimed asphalt in new roads, and reduce the embodied carbon of the binder. "Most of the focus at the moment is on carbon reduction in mixes, but there is also asset resilience, developing surfacing that lasts longer," says Edwards. The National Highways low-carbon asphalt programme is specifically aimed at road surfacing - typically the top 30-50mm of national network roads - but outcomes from this research are being fed directly into wider-ranging initiatives such as the 'near net zero' resurfacing project that took place on the A64 in Yorkshire last month (October). This scheme, being delivered by Tarmac, addressed carbon emissions in the wider highway construction work and used electric rollers, delivery vehicles running on hydrotreated vegetable oil and, quite controversially, closing the road entirely for joint-free surfacing to be laid to enhance durability.
The marketplace has changed significantly in the short time since the programme began in 2022, says James. "Innovations are appearing almost weekly; there is a huge amount of interest in this project and the industry is really keen to collaborate and to get these innovations understood and tested. People are really pushing in this space and they want to find solutions."
The strategy of focusing on market-ready technologies enabled the first trial to start within a few months of the research programme's launch. In March 2023 an 'off-network' trial saw mixes with biogenic binder substitutes laid on the A30 in Devon, leading to further trials of these materials on the A2 near Canterbury and the A34 near Newbury this summer.
"We had an opportunity to do an early trial, which enabled us to focus on this as a quick win. It was a direct substitution and we were able to go to network trial quite quickly," reports Edwards. In the more recent trials, recycled asphalt was brought into the mix and steps were taken to reduce the carbon footprint of the mixing process.
On the A34, the planed material from the old surfacing was fed directly back into the new pavement mix at quantities of up to 30%. And this was 'warm mix' asphalt which is made at a lower temperature and hence uses less energy. Edwards says that warm mixes are now the default for National Highways, one of the first innovations to be approved and introduced in recent years.
The current research is exploring the feasibility of 'half-warm' mixes to take this energy demand down even further. Such mixes are mostly used in warmer climates, and haven't had a lot of uptake in the UK. Modifications to plant are usually necessary.
"Half-warm has been trialled extensively in the UK for lower layers but not so much in surfacing," says James. "We would be looking specifically at introducing it into the top layers; we haven't got there yet but that will be our next step," she adds.
Another surfacing trial that has just started is investigating the durability properties of asphalt with a graphene additive. The additive, which is being tested in asphalt that was laid on the A12 in Essex in September, is intended to make the material more resistant to weather and vehicle damage, but researchers want to understand whether it will also give the material a longer life. "We want to be sure we don't make it too stiff so we don't get problems with cracking and so on, and we also need to make sure it's rut-resistant and won't flow under the action of traffic," explains James. Meanwhile on the A21, where the research team is experimenting with mixes using higher proportions of recycled asphalt, the introduction of a 'bio-rejuvenator' additive that claims to restore the properties of the aging bitumen is being explored.
This article was first published in the November 2024 issue of The Construction Index Magazine. Sign up online.[3] Clearly network trials are essential but they offer a long-term perspective and are only one part of the jigsaw - laboratory testing offers a faster assessment of mixes and the opportunity to compare them with what is currently in use.
Gordon Airey, head of pavement research at the University of Nottingham, says that it is not easy to establish whether new binders, for example, will give the same long-term result as conventional products. "Standard empirical tests aren't going to be that useful, as they won't give us any indication of performance and durability," he explains. "We are trying to capture those properties with tests that are more focused on failure mechanisms rather than just coming up with a number. That's the beauty of this project - we are working very closely with the people who are producing the material in the field, with those who are specifying the material, and those who are designing the material.
This collaborative effort has made it a lot easier for us to work out how the materials are going to perform." But homing in on the right mix is a balancing act, he adds. "Asphalt is aggregate that has been stuck together with bitumen; the tendency to use a crude oil-sourced binder is changing because we've got a better understanding of bio binders or other materials that are more carbon-friendly.
"Bio binders typically come from a range of different sources - waste cooking oils, engine oils or waste products from crops. Lignin, for example, has an ability to stick things together and we are trying to establish whether we can make use of those properties.
This is a great area of research at the moment - not just in the UK but also internationally." Lab tests so far have been encouraging, reports James: "The majority of the tests that we've done to date have been on mixes with bio-based binders, a higher recycled content and a warm mix additive, and they have all showed very comparable results with standard materials." Tests include wheel-tracking samples under water, soaking them in water, and subjecting them to repeated heavy loading - "in effect we 'torture' the asphalt," she says. Edwards regards National Highway's research work as a conveyor belt process intended to deliver a stream of innovations to the industry.
"In 2021 we introduced warm mixes, next year we are going to bring in mixes that permit a higher recycled asphalt content, and we expect to start drafting specifications next year for the work we are doing here. We're not going to wait five years to bring these things on to the network because 2040 is not far off." National Highways asphalt trial sites
A30 Cutteridge to Alphington in Devon: trials of asphalt with polymer-modified bio-binder A2 Kingston Bypass near Canterbury, Kent: trials of warm-mix asphalt with polymer modified bio-binder and reclaimed asphalt A34 East Ilsley, Oxfordshire to Chieveley, West Berkshire: trials of warm-mix asphalt with polymer modified bio-binder and reclaimed asphalt
A12 Hatfield Peverel to Witham, Essex: trials of proprietary graphene additive in warm- and hot-mix asphalt
A21 Back Lane along the Sevenoaks Bypass, Kent: warm-mix trials with high levels of reclaimed asphalt in straight run and polymer modified binder using bio-rejuvenator National highways low carbon investigations The emerging low carbon technologies being investigated by National Highways include:
Bio-binders Bio-binders replace the fossil-fuel based bitumen in asphalt with low-carbon industrial alternatives. Long-life binders
Long-life binders improve the lifespan of asphalt. Recycled plastic and crumb rubber additives These replace higher-carbon additives in the asphalt and repurpose waste materials such as old tyres.
Graphene additives Using processed mineral graphite to make the asphalt more resistant to weather and vehicle damage. High reclaimed asphalt
Recycling a higher proportion of asphalt Bio-rejuvenators Additives that make reclaimed asphalt behave more like virgin asphalt.
Half-warm asphalt technologies Reducing both the temperature that asphalt is laid at and the fuel needed to heat it. This enables roads to be open sooner after asphalt is laid.
This article was first published in the November 2024 issue of The Construction Index Magazine. Sign up online.[4] Hydrated lime - forgotten hero?
Not all the additives that improve asphalt durability and performance are recent arrivals on the scene. Hydrated lime has been used in asphalt mixes in the USA since the 1970s and while the mineral has been approved and occasionally used for surfacing in the UK, recent years have seen it overlooked in favour of new products. But according to Mike Haynes, director of MPA Lime, a specialist arm of trade body the Mineral Products Association), the durability of highway surfaces could be radically improved by adding a small amount of hydrated lime to asphalt mixes.
In fact the additive can benefit the asphalt in numerous ways: stiffening it, improving its rutting resistance, fatigue resistance and moisture resistance, and reducing the chemical ageing of the bitumen. Research carried out by MPA Lime suggests that with only a small amount of this overlooked additive, typically less than 1.5% by weight, asphalt could last up to 25% longer.
Such an increase in longevity might go some way to offsetting the carbon dioxide emitted when hydrated lime is made. It is already a permitted multi-functional additive in the UK's National Highways asphalt specifications and is in the register of durability improvement options being examined in the low carbon surfacing research programme.
Haynes says: "Recent reports from the National Audit Office and the Annual Local Authority Road Maintenance survey show that we need to spend a little bit more to get better durability and longer life out of road surfacing. "I believe asphalt could be a significant market for us, and that's why we are wanting to get the message out there: don't forget hydrated lime! We haven't been pushing it enough, but at the same time it has been forgotten by the industry.
Across the US and Europe it is used extensively, it just doesn't seem to be happening in the UK yet." This article was first published in the November 2024 issue of The Construction Index Magazine. Sign up online.[5]
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References
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